Typical applications of polymers, such as polyolefins, benefit from a polymer having useful properties over a wide range of temperatures and under a variety of conditions. For example, the ability to adjust the stiffness-toughness balance and processability of polyolefins is desirable to meet the needs of a broad range of applications at a lower cost. Stiffness-toughness balance may be adjusted, for example, by blending polymers and plasticizers. For instance, a plasticizer added to a high molecular weight, highly crystalline stiff polyolefin can soften the structure to improve the toughness of such a material. Plasticizers with low glass transition temperature also can extend the flexibility of plastics to lower temperatures by lowering the glass transition temperature of the polymer-plasticizer blend. Plasticizers are also beneficial during polymer processing due to improvements in a number of characteristics, such as lubricity, viscosity, ease of fusion, etc. The concept of plasticization, the benefits of using plasticizers, and the different methods of using plasticizers are described in J. K. Sears, J. R. Darby, The Technology of Plasticizers, Wiley, New York, 1982, which is directed to the plasticization of poly(vinyl chloride).
Many polymers such as ethylene-propylene rubber or ethylene-propylene-diene (EPDM) rubber products, for example, contain mineral oil as a plasticizer. Blending (e.g., in-line) of mineral oil with a high molecular weight EPDM polymer often significantly reduces the Mooney viscosity of the product relative to that of the polymer. This generally leads to efficient finishing of the oil-extended polymer and also facilitates compound mixing, extrusion, and other processing of the product. However, due to the often large loadings of extension oil in these products, extension oil is typically stored and transported in bulk (e.g., in railcars or tank trucks). While the freshly produced oils may be low in oxygen and moisture content, bulk storage and transport operations generally do not prevent exposure to air and moisture, thereby leading to potential contamination. For example, bulk storage tanks for low-volatility liquids are generally vented directly to the atmosphere, or are equipped with pressure-vacuum vents that draw in atmospheric air when pressure drops below a set value (as when liquid level drops). Tank trucks may be similarly equipped. Further exposure can also occur as loading and unloading equipment (e.g., flexible hoses) are attached to the bulk transport equipment. The oils also tend to contain a fraction of relatively volatile components (e.g., normal boiling point below 120° C.), including sulfur compounds, nitrogen compounds, and oxygenates, that can affect polymer production processes.
Polymer production processes are often sensitive to impurities such as moisture and oxygen. For example, in a solution metallocene process for making polymers such as EPDM rubber, the presence of oxygen in the polymer finishing section can generate undesirable product gels due to free-radical polymer chain cross-linking. Also, introduction of moisture in the recycle purification phase can shorten adsorbent bed life and cause icing in equipment upstream that is operated at low temperature. In addition, oxygen, moisture, and other nucleophiles (e.g., oxygenates, sulfur compounds, nitrogen compounds, halogenated compounds) are highly reactive with metallocene catalysts (and many other catalyst types), which could negatively affect reactor control, product quality, and plant efficiency.
Many plasticizers contain moisture, oxygen, reactive impurities, polymer gels, and solids as a result of the original manufacturing process or from exposure or cross-contamination during storage and handling. When added to a polymerization process, some impurities may cause undesirable impact on product quality or process efficiency. In other situations, the impurities may partition into multiple process streams, react with other stream components to form other reactive impurities, or concentrate within the polymerization plant over time, thus causing undesirable results.
It can be costly and inefficient to remove impurities after introduction to a polymerization process. For instance, multiple streams may require purification, impurities may be at a low concentration, properties of impurities and other stream components may be very similar, or the process stream properties (e.g., high viscosity, high pressure) may complicate the use of conventional purification technologies. It can also be costly and of limited effectiveness to mitigate the effects of impurities through actions, such as adding product stabilizers, installing gel filters, or changing equipment metallurgy.
U.S. Pat. No. 7,910,637 describes a process for in-line blending of polymers and plasticizers in a polymerization plant, i.e., before the recovery and pelletizing of the one or more solid polymer blend components.
U.S. Pat. No. 8,221,524 describes a method for achieving low oxygen levels in a natural gas stream without the use of a catalytic system. For example, this reference describes a method using membrane treatment for the removal of the bulk of CO2 and oxygen in the natural gas feed and the addition of a pressure swing adsorption (PSA) system using a carbon molecular sieve adsorbent for the adsorption of residual oxygen.
U.S. Pat. No. 8,388,740 describes a method for removal of dissolved oxygen from hydrocarbon streams. For example, this reference describes a process which includes mixing hydrocarbons streams with an oxygen free gas to form a gas-liquid mixture; the mixture is allowed to disengage into a gas stream and a liquid stream, thereby removing dissolved oxygen from the hydrocarbon stream.
Additional related background references on polymer production process and purification include: U.S. Pat. No. 7,163,989; P. S. Ravishankar, “Treatise on EPDM”, Rubber Chemistry and Technology, 85, 327-349 (2012); Perry's Chemical Engineers' Handbook, 8th Edition (2007); and “UOP Adsorbents: Purification of olefin and polymer process streams,” UOP LLC, July, 2011.
There is a need for an effective and efficient process for removing impurities from plasticizers before adding them to polymers in a polymer production process (e.g., via in-line blending). The present invention addresses this need by providing a process for purifying plasticizer particularly for use in a polymer production process and plant.